Field of the Invention
The invention concerns a method for acquiring a magnetic resonance imaging dataset of an area to be examined of a patient by operation of a magnetic resonance apparatus, as well as a magnetic resonance apparatus for implementing such a method.
Description of the Prior Art
Magnetic resonance imaging has become established for the examination of patients in medical practice. One frequently discussed problem is the measuring time required for the acquisition of magnetic resonance data. Depending upon the resolution, the acquisition of a magnetic resonance imaging dataset, in particular an at least three-dimensional magnetic resonance dataset, can necessitate a measuring time of from a few milliseconds to several minutes. The higher the desired resolution, the longer the estimated acquisition period. High resolution is necessary in the case of orthopedic diagnoses, for example, for the evaluation of cartilage damage, tendon injuries or ligament injuries. In such cases, the contrast required can result in measuring times ranging from 3 to 10 minutes.
Various possibilities are known for reducing the acquisition period, by the term “accelerated imaging”. Examples of this include parallel imaging, see for example the article by M Griswold et al, “Generalized autocalibrating partially parallel acquisitions (GRAPPA)”, Magn. Reson. Med. 47 (2002), pages 1202-1210, simultaneous multi-slice imaging, see for example in this regard the article by E. J. Larkman et al., JMRI 13 (2001), pages 313-317, or the article by K. Setsompop et al., MRM 67 (2011), pages 1210-1224, and simultaneous imaging refocusing, see for example in this regard U.S. Pat. No. 6,614,225 B1. However, with all these methods, the result of the measurement is only visible following complete data acquisition and it is only at this time that it is possible to evaluate whether the measurement has to be repeated, for example due to a motion of the patient. This extends the duration of the examination.
Since, it is not known prior to a measurement whether the image quality is excessively impaired by the patient motion, it is not possible to predict the overall duration of the examination and so patients' appointments have to be planned correspondingly generously. Without generous planning of this kind, delays in the time schedule may occur. In the first instance, fewer patients can be examined and additional costs are incurred since the magnetic resonance system is not permanently used at full capacity. In the second case, patients are subjected to waiting times and this has a negative impact on satisfaction with the medical service.
A further problem relating to motions that occur during the acquisition of magnetic resonance data is that even the operator present at the magnetic resonance mechanism during the examination is unable to determine in all measurements whether the patient moves. Only specific magnetic resonance sequences, for example for neurofunctional imaging, measure motion during the acquisition of magnetic resonance data.
To date, on the occurrence of a motion in the area of the patient to be examined, it has only been decided at the end of the measurement, that is when the magnetic resonance imaging dataset is available, whether a new measurement, therefore a new acquisition of magnetic resonance data from the patient, is necessary.
In order to achieve improvements in the light of this problem, US 2014/0125335 A1 suggested a method with which the occurrence of motions is monitored during the measurement in order to stop the acquisition of data if necessary. Therefore, this publication relates to a method for magnetic resonance imaging with which the acquisition process is divided into two parts, namely an essential component (“basic acquisition”) and an additional component (“complementary acquisition”) by means of which an improvement in image quality can be achieved until a motion occurs. The essential part of the acquisition is performed at the start and finally includes a sub-sampled basic dataset from which reconstruction would already be possible in principle. The further acquisition process of indeterminate length supplements this data. In this case, only one maximum measuring time is disclosed. If a motion occurs in the essential part of this measurement, the entire measurement has to be repeated. Therefore, in this case, the motion monitoring relates to the motion status in the area to be examined at the start of the acquisition process.
However, the method described in US 2014/0125335 A1, facilitates neither improved planning, since it can still be the case that measurements have to be completely repeated and the acquisition period varied, nor reliably high image quality, since it is possible that the acquisition is aborted very early on in the complementary component of the measurement.